This invention relates to a method and an apparatus for continuously applying a coating of constant thickness to a web (sheet) of indefinite length with the aid of an elastic doctor blade, an edge of which presses, along a scraping line, the running web and which is, at a location spaced from that edge, firmly held by a clamping device.
To improve the quality of raw paper, it is known to coat base paper in a coating machine up to twice on each side with an appropriate substance. The substance is applied to the sheet in excess and subsequently, by means of a doctor blade, the coating is brought to the desired weight. It is a disadvantage of this conventional method that because of impurities in the coating mass, between the doctor blade and the sheet deposits may appear which result in streaks that adversely affect the quality of the paper.
U.S. Pat. No. 1,566,358 discloses an apparatus for tightening and biasing an elastic doctor blade wherein in the frontal part of the doctor blade at its side which is oriented away from the sheet a support strip with an elastic insert is provided. In order to be able to vary the pressure of the tip of the doctor blade on the sheet, the elastic insert is configured as a hose which contains a pressure medium and which is connected with a controllable pressure source. This reference does not address the problem of avoiding streaking.
German Patent No. 3,313,972 discloses an apparatus for the clamping and biasing of an elastic doctor blade of a coating apparatus. To set the bias between the clamping device and the support strip on that side of the doctor blade which is oriented towards the sheet, a specially shaped, pressure medium-containing hose is provided. The latter, upon pressurization, presses the doctor blade in the region between the clamping device and the support strip away from the sheet so that the tip of the doctor blade situated above the support strip is pressed against the sheet. This patent too, does not discuss the manner in which streaking may be avoided.